1. Field of the Invention
The present invention relates to a nucleic acid-bound polypeptide, a method of producing the nucleic acid-bound polypeptide, and an immunoassay using the nucleic acid-bound polypeptide.
2. Discussion of Background
Various studies have been made as to how to maintain the specific steric structure of a combinant protein produced by gene engineering, more specifically gene manipulation, and also as to how to apply the thus produced protein to an antigen-antibody reaction.
In the production of the combinant protein, in particular, in the course of a purification step of the produced protein, a denaturation operation is inevitably carried out. In such purification step, it is not always possible to maintain a natural structure of the protein, so that such protein cannot be used in an immunoassay system.
Various factors are also known that affect reactions which are peculiar to each of various assays. It is known that for these reasons or other, the above-mentioned antigen-antibody reaction does not always proceed as desired when the combinant protein is used.
For example, there is known an agglutination immunoassay as one of immunoassays. For instance, when an antibody corresponding to an antigen is assayed by agglutination immunoassay, the antigen is fixed on the surface of particles such as latex particles, and such antigen-fixed particles are allowed to react with the antibody in a test sample. When the antibody is present in the test sample, the antigen-fixed particles agglutinate due to the antigen-antibody reaction, so that, for instance, the absorbance of the test sample changes. Therefore by measuring the absorbance of the test sample, the degree of the agglutination can be determined, and accordingly the antibody in the test sample can be quantitatively measured from the measured absorbance of the test sample.
However, when the recombinant protein is used as the antigen to be fixed on the surface of the particles in the above-mentioned agglutination immunoassay, it occasionally occurs that even though the protein itself has reactivity with the antibody to be assayed and the antibody is in fact present in the test sample, no agglutination takes place.
Conventionally, in the case where no agglutination takes place as mentioned above, the recombinant protein is modified or expressed in the form of a fused protein in order to improve the agglutination reactivity of the protein. However, it is extremely difficult to modify the protein so as to impart the desired properties thereto, while maintaining the antigenicity (i.e. the reactivity with the antibody).
Furthermore, the recombinant protein is often of an insoluble kind, so that when the thus produced protein is purified, the protein has to be subjected to solubilization treatment. However, the protein is often denatured in the course of the purification treatment, losing the necessary antigenicity.
Therefore, it is preferable that a soluble protein be directly produced by genetic engineering.